The present invention relates to a low temperature vapor phase process for the selective preparation of acrolein, acrylic acid and mixtures thereof which comprises contacting propylene, water and oxygen with a solid catalyst comprising a catalytically effective amount of noble metal such as palladium promoted by a solid acid consisting of mixed metal oxides, such as a mixture of molybdenum and titanium oxides.
Acrolein and acrylic acid and methacrolein and methacrylic acid are very important chemicals and are particularly useful in the preparation of a variety of commercial products, including plastic sheet and molding powder for signs, emulsion polymers for water-based paint formulations, paper coating, and many other such applications.
A number of processes have been proposed for the preparation of these materials and are well-known in the art. See, for example, "Encyclopedia of Chemical Technology", Second Edition, Kirk-Othmer, Vol. 1, pages 293-295. Also, Chapter 10 of "Propylene and its Industrial Derivatives", by E. G. Handcock, published by John Wiley, New York, 1973.
In general, the preparation of acrolein and/or acrylic acid from propylene is a catalytic oxidation process wherein the catalyst is the important feature of the invention. Thus, U.S. Pat. No. 2,941,007 discloses a process for the production of acrolein from propylene by catalytic oxidation using a catalyst consisting of bismuth molybdate and bismuth phosphomolybdate. U.S. Pat. Nos. 3,435,069 and 3,595,911 show the catalytic oxidation of propylene or acrolein to acrylic acid using a catalyst consisting of a mixture of an oxide of molybdenum and an oxide of titanium and antimony, molybdenum and titanium oxides, respectively. These processes are not desirable however, since they must be operated at relatively high temperatures above about 250.degree. C., and preferably about 350.degree. C. As is shown hereinbelow in Example 3, a catalyst composition containing only oxides of molybdenum and titanium does not produce acrolein or acrylic acid when propylene is oxidized at, say, 152.degree. C. Thus, a need still exists for processes which are operable at relatively low temperatures, such as in the range of about 100.degree. C.-250.degree. C. and which are selective to acrolein and/or acrylic acid.
U.S. Pat. No. 3,792,086 (A) describes such a low temperature process employing a catalyst composition containing phosphoric acid and palladium metal in the preparation of acrylic or methacrylic acids by the vapor phase oxidation of propylene or isobutylene, respectively. A number of U.S. patents directed to the preparation of such products are noted therein. U.S. Pat. No. 3,947,495 (B) shows an improved catalyst composition containing a sulfur modifier, patents A and B being hereby incorporated by reference. These processes while useful to produce acrylic acid from propylene are not useful however, for the selective conversion of propylene to acrolein. Thus, for instance, Examples 1-10 in U.S. Pat. No. 3,792,086 produce only a small amount of acrolein relative to acrylic acid, e.g., an acrylic acid to acrolein mole ratio of from about 5.5/1 to 30/1. A similar run as shown hereinbelow provides an acrylic acid to acrolein mole ratio of about 1/5, thereby significantly changing the selectivity of the propylene oxidation process.
It is among the objects of the present invention to provide a new and improved low temperature process for the selective preparation of acrolein, acrylic acid and mixtures thereof from propylene by a direct and efficient vapor phase process.
It is a further object of the invention to provide a new catalyst composition and a method for making the catalyst.
Other objects and advantages of the invention will be apparent from the following description.